This invention relates to high performance data cables that successfully enables transmission in the frequency range of 0.3 MHz to 600 MHz. More particularly, I provide a helical shielded twisted pair cable with a standard impedance deviation of 3.5 or less about the mean or average impedance of 50 to 200 ohms. Also, I provide a high performance data cable having a plurality of the helical shielded twisted pair cables and having an average standard deviation of 3.5 or less and with no single standard deviation for any of the cables being greater than 4.5.
The current high performance data cables usually utilize as a shield a heavy, stiff, 2 mil aluminum tape with a 1 mil polyester (Mylar) backing. The shield is wrapped around each unshielded twisted pair subgroup within an application lay length that is equal to the length of the cables overall cable lay, typically lays of 4.0 to 6.0 inches. The tape is about 0.5 inches wide. The application angle of the wrapping is shallow, based on the long overall cable lay (5 inches) and the tape is almost parallel with the twisted pair laterally axis. A typical cable has 4 pairs of twisted pair cables with a 40 to 65% tinned copper braid applied over the four pairs and a final thermoplastic jacket extruded over the braided pairs to complete the cable. The shallow application angle of the metal shield tape generally creates the problem of allowing the tape to open up during the cabling operation before a binder or spirally applied drain wire can capture it.
Also, the tape doesn""t generally follow the pairs contour under the tape. Tape gaps are created with this process around the unshielded twisted pair core that do not provide a sufficiently stable ground plane to meet the industry standard electrical requirements such as CENELEC pr EN 50288-4-1.
The known cable structure noted above is mechanically unsound in a static state, and the electricals are unstable under installation conditions since the single overall braid cannot adequately insure the tape lap doesn""t xe2x80x9cflowerxe2x80x9d open when the cable is flexed. This xe2x80x9cfloweringxe2x80x9d increases NEXR, and further erodes impedance/RL performance as the ground plane is upset. This adds to attenuation nonuniformity. The impedance numbers are even worse under flexing since the conductor""s center to center, as well as the ground plane, changes. The higher the bandwidth requirement, the worse these issues become.
My invention uses a spiral wrap shielding tape to meet impedance/RL, attenuation uniformity, and capacitance unbalance that is required.
My invention eliminates most of the trapped air that is normally found in shielded twisted pair cables. This is done by helically or spirally wrapping the shield with a 25-65% and preferably a 45-55% overlap. The shield has a 0.33 to 2.0 mil and preferably close to 1 mil metal layer, i.e., 0.75 to 1.25 mils. The helical or spiral wrap with its overlap combine to provide good shielding with improved impedance control. The consistent ground plane created along the cables length allows better capacitance unbalance.
My invention also provides for substantial geometric stability under flexing. My use of short lay shield tapes eliminate tape gaps and flowering under flexing by using tapes with my preferred tape overlap of 45 to 55% overlap and an angle of wrap that is 30 to 45xc2x0 and no more than a 45xc2x0 relative to the cable""s longitudinal axis. This establishes a very stable level of physical and electrical performance under adverse use conditions. My twisted pair cable center to center distances indicated as (d) in FIG. 3, and conductor to ground distances, remain much more stable than those of the previous cables.
My cables are especially beneficial for use as category 7 and higher cables. This is especially true for those cables that I spirally or helically shield and are used out to 600 MHz. The typical high-performance data cable when made according to our invention, has four (4)twisted pair cables with each twisted pair cable made up of two foam or non-foam insulated (fluorocopolymer or polyolefin) singles. Each of the helical shielded twisted pair cables has my unique tight helical metal shield tape wrapped around it with the tape and its lateral short fold seam tightly held in place with a the tight 25 to 65% and preferably 45 to 55% overlap. The helical shielded twisted pairs are S-Z""d or planetary together into a bunched or bundled configuration. The bundled pairs may be bundled by an overall braid or threadxe2x80x94metal or fabric. A final thermoplastic jacket (fluorocopolymer or a polyolefin, i.e., polyvinyl chloride) is extruded over the bundled twisted pair cables.
Generally the metal shield is an aluminum tape or a composite tape such as a short fold BELDFOIL tape (this is a shield in which metal foil or coating is applied to one side of a supporting plastic film), or a DUOFOIL tape ( this is a shield in which the metallic foil or coating is applied to both sides of a supporting plastic film) or a free edge BELDFOIL tape. The overall metal thickness is 0.33 to 2.0 mil aluminum layer thickness and preferably about a 1.0 mil. Although aluminum is referred to, any suitable metal normally used for such metal and composite metal tapes can be used such as copper, copper alloy, silver, nickel, etc. Each twisted pair is wrapped with the metal facing outwardly and although the most preferred wrap is a 45 to 55% overlap. As noted above, the overlap may vary as a practical matter from 25 to 65%. The preferred shield that gives the best attenuation and impedance characteristics are those tapes that are joined to provide a shorting effect. However, with a suitable overlap, the short fold can be eliminated.
The number of shielded twisted pairs in a high performance data cable is generally from 4 to 8 but may be more if desired. The tension of the helically wrapped shield is such that the wrapped shield eliminates most of the trapped air to provide a standard impedance deviation for the helical shielded twisted pair cable and an average standard impedance deviation for the high performance data cable which has a plurality of helically shielded twisted pairs. The tension on the shielding tape and binder are such that there is only a 25% or less and preferably 18% or less void space of the entire cross-sectional area of the helical shielded twisted pair taken along any point in the length of the cable.
I provide a high performance twisted pair data cable having a shield helically wrapped around an unshielded twisted pair cable and if desired a fabric or metal braid or thread simultaneously or subsequently wrapped around the helical shield to additionally bind the shield. The wrapping of the shield and binder(the braid or thread) is at a tension such that for an individual twisted pair that may be used on its own, the individual pair has an unfitted impedance that has a nominal or standard impedance deviation of 3.5 or less for each helical shielded twisted pair cable that is rated for up to 600 MHz The high-performance data cable which has a plurality of helical shielded twisted pair cables and is rated at up to 600 MHz has an average standard impedance deviation for all of the plurality of helically shielded twisted pairs of 3.5 or less and with no single standard impedance deviation being greater than 4.5. The standard impedance deviation is calculated around a mean or average impedance of 50 to 200 ohms and preferably 90 to 110 ohms and with at least 350 frequency measurement taken on a 328 ft. or longer cable.
Other advantages of my invention will become more apparent upon reading the following preferred description taken in conjunction with the drawings.